The invention relates to a method for encoding a picture, a computer program product and an encoder.
Perceptual content-adaptive video/image communication (PCAVC) adapts the concept of visual attention into video compression. Human eyes are not perfect sensors. Some information is not visible to human eyes. The term “Just-Noticeable-Difference (JND)” indicates the tolerance that has a 50% possibility being able to be perceived by human eyes under certain conditions (visual sensitivity is the inversa of the modulated JND). Moreover, human eyes are also highly selective sensors. Their selectivity depends not only on local contents, but also global cognitive (high-level) contents. Their global selectivity reflects on the resource allocation competition on both low-level and high-level visual contents. The selectivity of human eyes is also called “visual attention”.
It is believed that the computational resources in the human brain are limited. With millions of years of evolution, the mechanism of visual attention has developed in the human brain, a set of strategies to reduce the computational cost of the search processes inherent in visual perception.
By this mechanism, a human brain allocates more resources to one or more areas of the human's visual field, which contain the most important contents to human eyes. Determining what content is more important than another is a competition process.
Visual attention is a very important and complex mechanism of the human brain. It brings many aftereffects. One of the most important aftereffects of visual attention is its modulatory effect on visual sensitivity. The characteristics of visual attention's aftereffect on visual sensitivity can be outlined as:                1. Generally, visual sensitivity in the area of high visual attention is high;        2. Visual sensitivity is also determined by local complexity of contents;        3. The difference of sensitivity thresholds between focus areas and out-of-focus areas can reach 9 dB in maximum;        4. The difference of the sensitivity is controlled by visual attention level, which is a result of both top-down stimuli (high-level contents, knowledge and experience) and bottom-up stimuli (low-level contents: contrast, color, orientation, etc);        5. Human eyes are not blind out of focus;        6. There usually exists a gradient for the sensitivity thresholds between focus areas and out-of-focus areas;        7. The gradient width usually is flexible and it is controlled by the visual attention level in a focus area; when the visual attention level is high, the gradient width is small, and vice versa;        8. The movement of object brings motion suppression:                    a. motion suppression usually only exists in out-of-focus areas;            b. motion suppression is usually strong when the moving offset is big;motion suppression can reach 6 dB in maximum.                        
Region of interest techniques are known according to prior art, see for example [3]. For example, an image is coded and transmitted in such a way that for a certain part of the image (the region of interest) a better visual quality is achieved than for the rest of the image.
The existing region of interest techniques have the following shortcomings:                1. The distortion inside the region of interest is zero (i.e. lossless compression is used for the region of interest, cf. [3]), and the information of an image corresponding to areas outside the region of interest may totally be lost in low and very-low bitrate conditions.        2. The ratio of distortion between the inside and the outside of the region of interest cannot be controlled.        
An object of the invention is to provide an enhanced compression method compared to the compression methods according to prior art.